It is known that molds and cores, which are used in the casting of metal articles, can be made from a foundry aggregate, e.g. sand, and heat curable or no-bake foundry binders, e.g. furan binders. One of the problems with using heat curable binders for making cores and molds is that the process is slow, i.e. low productivity, and energy requirements are high. Another problem with using such binders is that the binders typically contain free formaldehyde and/or free phenol, which are undesirable from a health and safety standpoint. Because of these problems, there have been attempts to improve the quality, productivity, performance, and environmental acceptability of processes that use heat curable binders for making molds and cores.
Two of the best-known processes for making molds and cores with heat curable binders are the hot-box process and the warm-box process. The hot-box process uses a binder composed of phenolic and/or urea/formaldehyde resins, sometimes modified with furfuryl alcohol. The binder is mixed with a foundry aggregate and cured with latent, acid salt catalysts, such as ammonium chloride. Although the process provides thermally stable cores with high immediate and final strength, the process has disadvantages because there are significant amounts of free formaldehyde and free phenol in the binder.
Although there are some similarities between the warm-box process and the hot-box process, the warm-box process uses much higher levels of furfuryl alcohol than the hot-box process, and uses stronger latent acid salts and/or acids as curing catalysts than used in the hot-box process. Additionally, lower tooling temperatures are sometimes possible if the warm-box process is used. The curing chemistry of this process relies more on the acid curing of furfuryl alcohol to achieve the required reactivity and strength. Phenolic and urea/formaldehyde resins are generally still incorporated into the binder composition at lower levels, so the presence of free formaldehyde and phenol can still be a health and safety issue. The thermal stability of these binder systems is generally considered to be lower than hot-box binders because of the reduced amount of phenolic and urea/formaldehyde resins that impart hot strength. In addition there is often a significant compromise between reactivity and immediate strengths versus the working life of the mixed sand. By increasing the strength of the acid curing catalyst, the process can be carried out without heat, i.e. by the no-bake furan process. But because heat is not used, it is usually necessary to use acid curing catalysts having a greater strength. Typically, these catalysts are sulfur-containing catalysts, e.g. sulfuric acid, sulfonic acid, etc. The problem with using these sulfur-containing catalysts is that high levels of sulfur dioxide are typically emitted when metals are cast from the cores and molds made by the no-bake process. This has the potential of creating environmental, health, and safety issues.
Hot-box and warm-box binders often contain a urea/formaldehyde resin. These binders contain nitrogen that can be emitted as a gas during the casting process. The nitrogen gas emitted can cause casting defects if present at high levels, or the metal cast is sufficiently sensitive to this type of defect.
It is clear that there are advantages and disadvantages to the hot-box and warm-box processes. But both processes have a major disadvantage in common, which is the use of binders that contain free formaldehyde and free phenol to some degree. A heat-curable binder that did not contain free formaldehyde or free phenol would offer obvious advantages. In addition, if the no-bake process were used, reduced catalyst quantities or significantly weaker and/or lower sulfur content catalysts could be used instead of the typical addition or strength of curing catalysts used, which would typically result in lower sulfur dioxide emissions during the casting process.
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